1. Field of the Invention
The present invention relates to a dielectric filter, and particularly to a dielectric filter in which a plurality of resonator cavities which contain resonator holes having step portions are formed in a single dielectric block.
2. Description of Related Art
Conventionally, there has been known a dielectric filter in which a step portion is formed in a resonator cavity, forming two resonant portions having different line impedance, with the step portion at the boundary thereof, thereby obtaining a desired filter characteristic. FIGS. 1A and 1B show the construction of a conventional dielectric filter in which resonator cavities having step portions as described above are formed. FIG. 1A is a perspective view of the dielectric filter, taken from the open-circuited end surface of the dielectric filter, and FIG. 1B is a plan view showing the dielectric filter, also taken from the open-circuited end surface.
The dielectric filter shown in FIGS. 1A and 1B comprises a dielectric block 1 having a substantially rectangular parallelepiped shape, and resonator cavities 2a and 2b which are formed in the dielectric block 1, so as to penetrate through a pair of confronting surfaces of the dielectric block 1. As seen in FIG. 1A, each resonator cavity has an inner conductor 3 formed on the inner surface thereof. Further, input/output electrodes 5 are formed on the outer surface of the dielectric block 1, and an outer conductor 4 is formed substantially over the whole surface of the dielectric block 1 except for the areas at which the input/output electrodes 5 are formed.
At one end surface 1a of the dielectric block 1 (hereinafter referred to as the "open-circuited end surface"), a portion at which no inner conductor 3 is formed (hereinafter referred to as the "non-conductor portion"), is provided at one end portion of each of the resonator cavities 2a, 2b which is in the neighborhood of the end surface 1a as shown in FIG. 1A. Thus the inner conductor 3 formed in each resonator cavity 2a, 2b is separated from the outer conductor 4 by this non-conductor portion. On the other hand, at the opposite end surface 1b (see FIG. 1A) of the dielectric block 1 (hereinafter referred to as the "short-circuited end surface), each inner conductor 3 is short-circuited to the outer conductor 4.
In the dielectric filter thus constructed, a step portion 21 (see FIG. 1B) is provided substantially at the center portion between the open-circuited end surface 1a and the short-circuited end surface 1b in each resonator 2a, 2b, and these resonators 2a and 2b are designed so that the inner diameter thereof at the end surface 1a is larger than that at the end surface 1b. Hereinafter, the portion of each resonator cavity 2a, 2b which has the larger inner diameter is referred to as the "large inner-diameter portion", and the other portion of the resonator cavity which has the smaller inner diameter is referred to as the "small inner-diameter portion".
In this structure, the large inner-diameter portion is formed at the side of the open-circuited end surface 1a, and the coupling between both resonators is ordinarily strong capacitive coupling, so that a filter characteristic having a broad pass band and an attenuation pole at a low side of the pass band is obtained. Further, the resonant frequency of each resonator which is formed in each resonator cavity 2a, 2b, and the coupling degree of the resonators, can be varied by changing the ratio of the length of the large inner-diameter portion and the length of the small inner-diameter portion of the resonator cavity 2a or 2b and the ratio of the inner diameters of the resonator cavities 2a and 2b, thereby obtaining a desired filter characteristic.
However, in the conventional dielectric filter as described above, the large inner-diameter portions and the small inner-diameter portion of the resonator cavities 2a, 2b are designed to have a circular cross-sectional shape, and the center axes thereof are disposed coaxially. This places restrictions on the self-capacitance which is formed between the inner conductor 3 and the outer conductor 4, and the mutual capacitance which is formed between the neighboring inner conductors 3, and thus the degree of freedom in the design of a desired filter characteristic is low. That is, it is difficult to obtain various filter characteristics in a dielectric block 1 having a required body size. In other words, it is difficult to design the dielectric block 1 so that it has a desired body size and also to obtain required filter characteristics.